Characterisation of a megavolt X-Ray therapeutic beam using non-orthogonal strip detectors for future enhanced primary standard dosimetry

The National Physical Laboratory (NPL) provides traceable high accuracy measurements for absorbed dose via primary standard graphite calorimeters. Patients undergoing radiotherapy for cancer treatments in the U.K. receive a dose that is traceable to these devices. In the case of hadron radiotherapy, reducing the uncertainty of these measurements requires the addition of spatial information to understand not only the integral dose delivered, but where the energy is deposited locally. The PRaVDA consortium developed strip detectors able to track individual protons at low rates for proton computed tomography. For calorimetry at clinical dose rates only real-time spot location is required, which could be projected to reconstruct a 2D visualisation from which the beam centre can be determined. This work reports on a proof-of-concept trial carried out using 6 MV X-rays using an Elekta Synergy linear accelerator. We show how the beam positions can be determined within ± 0.5 mm of the expected radiation beam position, which is similar to the tolerances of the field defining multileaf collimator within the linear accelerator treatment head. The trial will be extended to perform measurements in a clinical proton beam alongside the NPL graphite proton calorimeter.